Method for improving the performance of a coding in a radio communication system

ABSTRACT

The invention relates to a method for improving the performance of data transmissions over a communications channel, wherein data is coded and modulated for transmission, wherein coding said data results in differently coded bits. In order to allow for a simple way of a more flexible coding, it is proposed that said differently coded bits are mapped for modulation to different modulation symbols of a symbol alphabet, to which modulation symbols different relative reliabilities are assigned, and wherein by mapping said coded bits to said modulation symbols, different reliabilities are associated to differently coded bits depending on the conditions on said communications channel. The invention equally relates to a corresponding communications system and to elements of such a communications system.

FIELD OF THE INVENTION

[0001] The invention relates to a method for improving the performanceof data transmissions over a communications channel, in particular theair interface in a radio communications system, wherein data is codedand modulated for transmission. The invention equally relates to a radiotransmission unit and to an encoder for such a radio transmission unit,to a radio receiving unit and to a decoder for such a radio receivingunit, and finally to a communications system comprising at least such aradio transmission unit.

BACKGROUND OF THE INVENTION

[0002] For radio communications systems, in which data is transmittedover the air interface from a transmitting unit to a receiving unit, itis known from the state of the art to transmit the respective dataeither in uncoded or in coded form.

[0003] Coding of to be transmitted data is used in order to be able todetect and/or correct the errors occurring in the transmission of dataon the transmission path on the air interface. In systematic codingmethods, e.g., for all data, systematic and parity bits are created. Thesystematic bits correspond to the data in uncoded form and the paritybits constitute additional, coded redundant information. For a codingrate of ⅓, for example, a systematic encoder provides for eachsystematic bit two parity bits.

[0004] Turbo coding is a systematic coding method that uses recursive,systematic convolutional codes. The parity bits are provided bydifferent constituent encoders using different constituent polynomials.The decoding is carried out iteratively in order to obtain a good errorcorrection performance.

[0005] Parallel concatenated turbo codes differ in the way theconstituent polynomials are defined and in the way interleaving is done.The polynomials are searched for a given operating point, bearing inmind complexity requirements. As in the other known coding methods, allencoded bits are transmitted with equal power. In case the actualoperating point differs from the operating point for which thepolynomials were determined, e.g. because of changed channel conditions,the fixed polynomials do not result any more in the best performance ofcoding.

[0006] It is known to vary an employed code by a method calledpuncturing, according to which every nth bit of a code is left out, thusreducing the coding rate, but this allows only for a limited adaptationto channel conditions.

[0007]FIGS. 1 and 2, which were extracted from “Turbo Coding” by ChrisHeegard and Stephen B. Wicker, Kluwer Academic Publishers, oppose fortwo different turbo decoding methods the performance obtained withtransmitted coded data and the performance obtained with transmitteduncoded data.

[0008] In both figures, the BER (bit error rate) is depicted over theEb/No (Eb=energy per bit; No=noise power density per Hz). Four curveswith asterisk illustrate in each figure the performance with coded databased on different numbers of iterations in decoding. The number next toeach curve indicates the number of iterations carried out. A furthercurve-without asterisk illustrates in each figure the performance withuncoded data.

[0009]FIG. 1 is based on serial turbo decoding. It can be seen that theBER of the uncoded data is better than the BER of coded datairrespective of the number of iterations for a Eb/No below approximately1.1 dB. For higher Eb/No, in contrast, the BER of the coded data isbetter than the BER of the uncoded data, the respective threshold valueof the Eb/No depending on the number of iterations used for decoding.

[0010]FIG. 2 is based on parallel turbo decoding. As can be seen in thediagram, the threshold value of the Eb/No for which the performance withuncoded data is better than with coded data depending on the number ofiterations is reduced, but in particular for lower numbers ofiterations, there still exist a range of better channel conditions, inwhich uncoded data results in a better performance.

[0011]FIGS. 1 and 2 therefore illustrate that the conventional turbocoding methods are not suited to enable an optimal coding performancefor all channel conditions.

SUMMARY OF THE INVENTION

[0012] It is an object of the invention to provide a method, a radiotransmission unit and a transmission encoder that allow for a simple wayof a more flexible coding.

[0013] This object is reached on the one hand with a method forimproving the performance of data transmissions over a communicationschannel, which can be for example the air interface. With the proposedmethod, the data is coded and modulated for transmission, wherein codingsaid data results in differently coded bits, i.e. in bits of at leasttwo different classes. One of these different classes may also compriseuncoded bits. The differently coded bits are mapped for modulation todifferent modulation symbols within the symbol alphabet, to whichmodulation symbols different relative reliabilities are assigned. By themapping of the coded bits to the modulation symbols, differentreliabilities are associated to the differently coded bits depending onthe conditions on said communications channel.

[0014] On the other hand, the object is reached with a radiotransmission unit for a radio communications system comprising encodingmeans for encoding data that is to be transmitted over a communicationschannel to a receiving unit outputting differently coded bits. Theproposed radio transmission unit further comprises modulating meansmapping the differently coded bits to different modulation symbolswithin the symbol alphabet, to which different modulation symbolsdifferent relative reliabilities are assigned. The mapping is carriedout and/or the modulation alphabet is modified according- to theproposed method of the invention. Transmitting means of the radiotransmission unit further take care of transmitting each modulationsymbol with the assigned reliability.

[0015] Moreover, the object is reached with a transmission encoder for aradio transmission unit of a radio communications system comprisingcorresponding encoding means and corresponding modulation means.

[0016] The object of the invention is also reached with a decoder for aradio receiving unit of a radio communications system comprising meansfor demodulating and decoding modulation symbols transmitted by theproposed radio transmission unit. The decoder has information about arespective association of employed relative reliabilities to differentlycoded bits in the radio transmission unit, and the means fordemodulating and decoding use this information in demodulating and/ordecoding the received modulation symbols. The object of the invention isalso reached with a radio receiving unit comprising such a decoder.

[0017] Finally, the object of the invention is reached with acommunications system comprising at least the proposed radiotransmission unit and a radio receiving unit.

[0018] The invention proceeds from the fact that the performance oftransmissions achieved with uncoded data can be better or worse than theperformance of a transmission achieved with coded symbols that are alltransmitted with the same transmission power or energy, depending atleast on part on the quality of the transmission path. The inventionproposes to introduce an additional parameter for coding and modulationthat can be used for taking account of the conditions on thetransmission path. This is achieved by enabling an unequal distributionof the relative reliabilities to different modulation symbols to whichdifferently coded bits are mapped. Thereby, also an unequal distributionof the reliabilities of the differently coded bits mapped to thesesymbols is obtained. The reliabilities assigned to the modulationsymbols can correspond to assigned relative effective transmissionpowers or different effective transmission energies.

[0019] If the distribution of different reliabilities to the differentlycoded bits is moreover adjustable, an adjustment of the transmissions tovarying channel conditions is enabled and thereby an enhancedperformance in decoding without having to touch a given code in anencoder of a transmitting end. If the reliabilities are changed withdifferent labelings, though, the modulator in the transmitter changesdepending on the channel quality. Labeling denotes any method in which asequence of input bits is associated with a particular output symbol,e.g. Ggay labeling. The reliabilities can be changed also by keeping thesignal labeling fixed, but by modifying the relative transmit powers,e.g. in the I and Q channels, depending on which bits need to have ahigher reliability. For example, with QPSK modulation equalreliabilities can be achieved with equal distance between eachconstellation point, while the relative reliabilities can be changed byusing an asymmetric modulation alphabet, in which the distance betweenconstellation points in the I-channel are further apart than those inthe Q-channel.

[0020] At the same time, also the decoder at the receiving end does notnecessarily have to be modified, since conventional weighting allows thereceiving end to operate without knowing the weighting patterns used atthe transmitting end. However, if the decoder knows the relativereliabilities these might be taken into account e.g. by changing thea-priori probabilities of the input bits differently for the bits withdifferent reliabilities.

[0021] The achieved enhancement depends on the chosen association ofrelative transmission powers to the differently coded bits and on thequality of the transmission path, e.g. on the effective instantaneous orlong-term average signal-to-noise ratio in the transmission chain. Theinvention thereby defines a parameterised coding concept, in which thecoding rate can remain fixed, but in which the reliability is allocateddifferently for at least two classes of bits, depending on the state orquality of the employed communications channel.

[0022] The precise power or energy allocation can of course depend alsoon other aspects, such as the encoding and modulation method used, andthe target quality of service point, e.g. the BER or FER (frame errorrate). The overall power employed for transmissions can be powercontrolled in any suitable way, while according to the invention,relative portions of this overall power are associated in additiondifferently to differently coded bits as a function of the channelstate.

[0023] The radio transmission unit can be for example a base station ora mobile station in its transmitting function and the radio receptionunit a base station or a mobile station in its receiving function.

[0024] Preferred embodiments of the invention become apparent from thesubclaims.

[0025] The association of different reliabilities to the differentlycoded bits can be realized in different ways. In a first preferredembodiment, the mapping of the differently coded bits to the modulationsymbols is fixed, and different relative reliabilities are assigned tothe modulation symbols depending on the channel conditions.

[0026] In a second preferred embodiment, the relative distribution ofthe reliabilities to the different modulation symbols is fixed, and thedifferently coded bits are mapped to the modulation symbols depending onthe current channel conditions.

[0027] A third preferred embodiment is a combination of the first andthe second preferred embodiment, in which both, the mapping and thedistribution of the reliabilities to the modulation symbols depends onthe current channel conditions.

[0028] The relative reliabilities can be controlled by the modulator.For example, different labelings for 16-QAM (quadrature amplitudemodulation) constellations can be employed, or different powers for 1and Q channels with QPSK (quadrature phase shift keying). Equally,different classes of bits may be transmitted in separate symbols, andthese separate symbols can be transmitted with a different relativetransmit power, for instance on orthogonal channels, using orthogonalcodes, using different carriers, different transmit antennas, or usingdifferent transmission times.

[0029] In another advantageous embodiment of the invention, a feedbackinformation is transmitted from a receiving end receiving transmittedmodulation symbols. This feedback information can either contain anindication of the present quality of the communication channel,represented e.g. by the SNR (Signal to Noise Ratio) or the fadingdistribution, etc. at the receiving end. Or the feedback information cancontain already an indication of the desired association of differentrelative reliabilities to said differently coded bits. The distributionof reliabilities to the different symbols of coded data, and thus thedistribution of different reliabilities to the differently coded bits,can then be carried out according to this feedback information.Alternatively or additionally, the radio transmission unit may measurethe channel state without explicit feedback, e.g. in TDD systems. Such alinking of the distribution of the power to the quality of thetransmission path constitutes an additional coding of the transmitteddata without changing the encoder itself.

[0030] Alternatively, in particular when there is no informationavailable at the transmitting end on the channel conditions, thedistribution of the relative reliabilities may be fixed. Thereliabilities can be fixed depending on the target SNR required to meeta given Quality of Service, like BER or FER etc., rather than onmeasured SNR. The target SNR depends on the channel fading distribution,on the actual coding and modulation concept used, and on the requiresbit or packet error rate.

[0031] The invention can be used in particular in combination withsystematic codings. In this case, the uncoded systematic bits can beassigned another reliability than the redundant or parity bits outpute.g. by constituent encoders. The systematic bits and the parity bitsare then the differently coded bits resulting in the coding of the data.

[0032] In the case of a systematic coding like power coding, preferablythe reliabilities are distributed to systematic and parity bits in a waythat the worse the channel conditions are, the more reliability isassociated to the systematic bits, since bad channel conditions resultin little coding gain. With particularly bad channel conditions, morereliability is associated to the systematic bits than to the paritybits. On the other hand, the reliabilities are associated with an equaldistribution to the systematic and the parity bits or a higherreliability is associated to the parity bits, in case of particularlygood channel conditions. The association of relative reliabilities tosystematic and redundant bits in between the two extremes of the channelconditions can be in between the associations of relatives reliabilitiesfor those extreme conditions.

[0033] By weighting the redundant bits higher than the systematic bits,steeper BER curves can be obtained for high SNR at the expense ofperformance at low SNR. This may even permit the use of shorter framesthan with ordinary turbo coding.

[0034] Exact patterns for associating relative reliabilities can forexample be found similarly to finding the best puncturing patterns.

[0035] In case that a plurality of parity bits are provided by anemployed systematic coding, alternatively or additionally to weightingthe systematic and parity bits differently in the association ofdifferent relative reliabilities, the parity bits may also be weighteddifferently among each other. It is known, e.g. from the above citeddocument “Turbo Coding”, page 103, that serial turbo decoders favour theconstituent decoder that is activated first, thus falsifying the result.With the method of the invention, this can be compensated already at thetransmitting end by associating different relative reliabilities to theparity bits.

[0036] The modulation means of the radio transmission unit and of thetransmission (TX) encoder of the invention can be suited to employ anyassociation of relative reliabilities to differently coded bits proposedfor the method of the invention.

[0037] The invention can be employed in particular, though notexclusively, with CDMA (Code Division Multiple Access).

[0038] In the paper “Tradeoffs Between Rate, Puncturing andOrthogonality in Space-Time Block Codes”, IEEE International conferenceon communications, Helsinki; June 2001, by O. Tirkkonen and A. Hottinen,a multi-antenna transmission concept was considered, in which differentsymbols are transmitted with different power in order to achieve a powerbalancing. In order to obtain equal BER for all bits, it was proposed touse different powers for QPSK and for 16-QAM. The invention can also beemployed to improve the results of this paper, e.g. by allocatingdifferent bits to QPSK symbols and to 16-QAM. For example, if thetransmit powers of each symbol is the same, the systematic bits can beallocated to QPSK symbols when operating at low SNR, while the paritybits are allocated to 16-QAM

BRIEF DESCRIPTION OF THE FIGURES

[0039] In the following, the invention is explained in more detail withreference to drawings, of which

[0040]FIG. 1 illustrates the performance of turbo decoding in serialmode;

[0041]FIG. 2 illustrates the performance of turbo decoding in parallelmode; and

[0042]FIG. 3 shows simulation results of the performance of coding withand without weighting according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0043]FIGS. 1 and 2 have already been described above.

[0044]FIG. 3 illustrates the performance of a coding that can beachieved with coded data weighted according to an example of the methodof the invention. The depicted performances are based on a simulation.

[0045] The described example of the method of the invention is based ona conventional ⅓ serial turbo coding employed in a radio communicationssystem. The system comprises a radio transmission unit with a turboencoder and a radio receiving unit with a turbo decoder. The turboencoder comprises two constituent encoders, each using a constituentpolynomial that was optimised for a given operating point.

[0046] The turbo encoder receives data that is to be transmitted to thereceiving unit. The data is processed in the two constituent encoders byapplying the different constituent polynomials. The turbo encoderoutputs in sequence one systematic bit that corresponds to an uncodeddata bit and two parity bits, each constituting the output of adifferent one of the two constituent decoders. The thus coded data isthen transmitted with the currently available total power for eachtriplet of bits to the radio receiving unit, using an interleaver sizeof 320 bits. The Eb/No at the receiver is supposed to be 1 dB.

[0047] The radio receiving unit receives the coded data and supplies itto the serial turbo decoder. The turbo decoder applies 9 Log-MAPiterations to the received coded data, the result of each iterationbeing available.

[0048] For the simulation, first equal weights [111] were assigned toall bits, where the first value is the weight for the systematic bit andthe second and third value the weight for the first and the secondparity bit. The bits were then transmitted with the transmission powerdistributed to the bits according to these weights. This corresponds toa conventional transmission with equal transmission power for allsystematic and parity bits.

[0049] Next, different weights [0.93 1.03 1.03] were assigned to thebits, the first value being again the weight for the systematic bit andthe second and third value the weight for the first and the secondparity bit. This means that about 10% higher weights were assigned tothe two parity bits than to the systematic bit of the ⅓ code. Accordingto these weights, the parity bits were then transmitted with atransmission power about 10% higher than the transmission power withwhich the systematic bits were transmitted.

[0050] In FIG. 3, the BER resulting in decoding for the two differenttransmissions is depicted over the number of turbo iterations carriedout. The BER resulting from the transmission with the total powerdistributed equally according to the equal weights is shown in FIG. 3 asa curve with a circle at every iteration step. The BER resulting fromthe transmission with the power distributed according to the differentweights is shown in FIG. 3 as simple line. As can be seen, with a singleturbo iteration, the BER is equal for both weightings, but with everyadditional iteration the BER of the curve resulting from the weightedtransmission decreases much more rapidly than the BER resulting from theunweighted transmission.

[0051] The two curves in FIG. 3 therefore illustrate that theperformance of a coding can be influenced significantly when employingthe method according to the invention by distributing the availabletotal transmission power differently to different symbols of a code.This implies that the proposed weighting allows to tailor the codeperformance without any increase of complexity in the transmitter or inthe receiver.

1. Method for improving the performance of data transmissions over acommunications channel, wherein data is coded and modulated fortransmission, wherein coding said data results in differently codedbits, wherein said differently coded bits are mapped for modulation todifferent modulation symbols of a symbol alphabet, to which modulationsymbols different relative reliabilities are assigned, and wherein bymapping said coded bits to said modulation symbols, differentreliabilities are associated to differently coded bits depending on theconditions on said communications channel.
 2. Method according to claim1, wherein the different relative reliabilities of said modulationsymbols are given by the assignment of a different relative transmitpower or a different relative effective transmit energy.
 3. Methodaccording to claim 1 or 2, wherein for associating differentreliabilities to the differently coded bits depending on the channelconditions, varying different relative reliabilities are assigned tosaid modulation symbols depending on the channel conditions.
 4. Methodaccording to claim 1 or 2, wherein for associating differentreliabilities to the differently coded bits depending on the channelconditions, said differently coded bits are mapped to said modulationsymbols depending on the channel conditions.
 5. Method according toclaim 4, wherein the assignment of the relative reliabilities to themodulation symbols is fixed or adjustable.
 6. Method according to one ofthe preceding claims, wherein for enabling an association of differentreliabilities to said differently coded bits depending on the channelconditions, a feedback is transmitted from a receiving unit receivingsignals on said communications channel.
 7. Method according to claim 6,wherein said feedback information contains an indication of theconditions on said communication channel.
 8. Method according to claim6, wherein said feedback information contains an indication of thedesired association of different relative reliabilities to saiddifferently coded bits, which indication is generated based on theconditions on said communication channel determined by said receivingunit.
 9. Method according to one of the preceding claims, wherein theemployed coding is a systematic coding resulting in uncoded systematicbits and parity bits, and wherein said systematic bits and said paritybits constitute said differently coded bits.
 10. Method according toclaim 9, wherein the relative reliabilities associated to saidsystematic bits is different from the relative reliabilities associatedto said parity bits.
 11. Method according to claim 10, wherein therelative reliabilities associated to said systematic bits are the higherthe worse the channel conditions are.
 12. Method according to one ofclaims 9 to 11, wherein the coding of said data results in at least onesystematic bit and at least two parity bits respectively, and whereinthe relative reliabilities associated to the parity bits is distributeddifferently to the different parity bits with a weighting suited tocompensate for a bias in decoding for at least one of the parity bitscompared to at least one other of the parity bits.
 13. Method accordingto one of the preceding claims, wherein the employed coding is turbocoding.
 14. Radio transmission unit for a radio communications systemcomprising: encoding means for encoding data that is to be transmittedover a communications channel to a receiving unit outputting differentlycoded bits; modulating means mapping said differently coded bits todifferent modulation symbols of a symbol alphabet, to which differentmodulation symbols different relative reliabilities are assigned,according to one of the preceding claims; and transmitting means fortransmitting each modulation symbol with the assigned reliability. 15.Radio transmission unit according to claim 14, further comprisingreceiving means for receiving a feedback information from a receivingunit receiving signals on said communications channel, on which feedbackinformation the association of the relative reliabilities todifferently-coded bits is based.
 16. Radio transmission unit accordingto claim 14 or 15, wherein the encoding means is a systematic encoder.17. Radio transmission unit according to claim 16, wherein the encodingmeans is a turbo encoder.
 18. Transmission encoder for a radiotransmission unit of a radio communications system comprising: encodingmeans for encoding data that is to be transmitted over a communicationschannel from a radio transmission unit to a radio receiving unit, saidencoding means outputting at least two differently coded bits; andmodulating means mapping said differently coded bits to differentmodulation symbols of a symbol alphabet, to which different modulationsymbols different relative reliabilities are assigned, according to oneof claims 1 to
 13. 19. Transmission encoder according to claim 18,wherein said encoding means is a systematic encoder.
 20. Transmissionencoder according to claim 19, wherein said encoding means is a turboencoder.
 21. Radio receiving unit for a radio communications systemcomprising a decoder with means for demodulating and decoding modulationsymbols of a symbol alphabet transmitted by a radio transmission unitaccording to one of claims 14 to 17, which decoder has information abouta respective association of an employed relative reliabilities todifferently coded bits in said radio transmission unit, wherein saidmeans for demodulating and decoding use said information in demodulatingand/or decoding said modulation symbols.
 22. Decoder for a radioreceiving unit of a radio communications system comprising means fordemodulating and decoding modulation symbols of a symbol alphabettransmitted by a radio transmission unit according to one of claims 14to 17, which decoder has information about a respective association ofemployed relative reliabilities to differently coded bits in said radiotransmission unit, wherein the means for demodulating and decoding usesaid information in demodulating and/or decoding said modulationsymbols.
 23. Communications system comprising at least a radiotransmission unit according to one of claims 14 to 17 and at least oneradio receiving unit, wherein said radio transmission unit uses acommunications channel for transmitting modulated symbols to said atleast one receiving unit.